Danks, A., Hall, S., & Schnepp, Z. (2016). The evolution of 'sol-gel'
chemistry as a technique for materials synthesis. Materials Horizons, 3(2),
91-112. DOI: 10.1039/C5MH00260E
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REVIEW
Cite this: Mater. Horiz., 2016,
3, 91
View Journal | View Issue
The evolution of ‘sol–gel’ chemistry as a
technique for materials synthesis
A. E. Danks,a S. R. Hallb and Z. Schnepp*a
From its initial use to describe hydrolysis and condensation processes, the term ‘sol–gel’ is now used for
a diverse range of chemistries. In fact, it is perhaps better defined more broadly as covering the synthesis
of solid materials such as metal oxides from solution-state precursors. These can include metal
alkoxides that crosslink to form metal–oxane gels, but also metal ion–chelate complexes or organic
Received 3rd November 2015,
Accepted 15th December 2015
polymer gels containing metal species. What is important across all of these examples is how the choice
DOI: 10.1039/c5mh00260e
review, we will attempt to classify different types of sol–gel precursor and how these can influence a
of precursor can have a significant impact on the structure and composition of the solid product. In this
sol–gel process, from self-assembly and ordering in the initial solution, to phase separation during the
www.rsc.li/materials-horizons
gelation process and finally to crystallographic transformations at high temperature.
1. Introduction
Since the discovery by early man that rocks could be modified
to make tools, the world has demanded materials with increasingly complex functionality.1 Given this fact, it is not surprising
that the development of new synthetic methods has been a
major field of scientific endeavour. Many inorganic materials,
a
School of Chemistry, University of Birmingham, Birmingham, B152TT, UK.
E-mail: [email protected]
b
Complex Functional Materials Group, School of Chemistry, University of Bristol,
Bristol, BS81TS, UK
A. E. Danks
Ashleigh Danks is currently studying
for a PhD at the University of
Birmingham in the group of Zoe
Schnepp. His research is focussed
on the sol–gel synthesis of nanocomposites of metal carbides, oxides
and carbon and he has a particular
interest in materials for catalysis.
During his PhD, he has spent several
months at the National Institute for
Materials Science in Japan as well
as a 6 month project working for the
Defence Science and Technology
Laboratory.
This journal is © The Royal Society of Chemistry 2016
such as metal oxides or carbides, can be prepared fairly simply
by mixing powder reactants and heating to give the desired
product. While reaction conditions are relatively easy to achieve
(furnace technology being well established), there are some
drawbacks. These centre primarily on the inhomogeneity of the
starting materials. In mixtures of two or more powders, complete
conversion is limited by mass transport. Initial reaction takes
place at the edges of adjacent particles and if reactant diffusion
is blocked there will be areas of unreacted starting material.
Some of these issues can be overcome by ball-milling: reducing
the particle size and increasing the sample surface area. However, extended heating or multiple treatments separated by
Simon Hall joined the University of
Bristol in 1997, reading for a PhD
in Biomimetic Materials Chemistry
in the laboratory of Professor
Stephen Mann FRS. After a three
year postdoctoral study, where he
produced advanced functional
materials for Vectura Ltd and
Toyota Ltd, he was awarded a
Royal Society University Research
Fellowship in 2004. His research
interests centre around crystal
control in superconductors, semiS. R. Hall
conductors, piezoelectrics and
polyaromatic hydrocarbons. He is currently a Senior Lecturer in
Inorganic Chemistry at the University of Bristol, Director of the
Complex Functional Materials Group and Co-I of the Bristol Centre
for Functional Nanomaterials Centre for Doctoral Training.
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Review
successive sample milling steps may be required. Furthermore,
it is often difficult to control particle morphology using solidstate methods.
As an alternative to solid-state chemistry, a range of solution
techniques have emerged, including coprecipitation, hydrothermal
processing, solvothermal methods and sol–gel chemistry.2 Among
these techniques, sol–gel chemistry offers some particular advantages, centred on the ability to produce a solid-state material from a
chemically homogeneous precursor. By trapping the ‘‘randomness
of the solution state’’ 3 and thereby ensuring atomic level mixing
of reagents, one should be able to produce complex inorganic
materials such as ternary and quaternary oxides at lower processing temperatures and shorter synthesis times. Furthermore,
sol–gel chemistry should enable greater control over particle
morphology and size. In reality, producing a homogeneous
precursor at room temperature does not ensure homogeneity
throughout a reaction and many sol–gel routes have therefore
been designed to combat or control phase segregation during
synthesis. Neither is it always necessary to ensure complete
‘randomness’ in the precursor. In fact some of the most interesting advances in the sol–gel field in recent years have come
from gels that have some degree of ordering and structure.
This review will give an introduction to the different types
of gel and then describe the types of chemistry that may be
considered under the heading ‘sol–gel’. We focus in particular
on how different molecular precursors can influence composition and structure in the synthesis of materials and highlight
mechanistic studies that have offered insight into the processes
that occur during sol–gel synthesis.
1.1
‘Sol’ and ‘gel’
Sol–gel chemistry is the preparation of inorganic polymers or
ceramics from solution through a transformation from liquid
precursors to a sol and finally to a network structure called a
‘gel’.4 Traditionally, the formation of a sol occurs through
hydrolysis and condensation of metal alkoxide precursors
Materials Horizons
(section 1.2), but a sol can be more generally defined as a
colloidal suspension, which encompasses a wider range of
systems. The International Union of Pure and Applied Chemistry (IUPAC) define a colloidal system as a dispersion of one
phase in another where, ‘‘the molecules or polymolecular
particles dispersed in a medium have at least in one direction
a dimension roughly between 1 nm and 1 mm’’. In this sense,
the term ‘sol’ can be applied to most of the systems discussed
in this paper, including in situ formation of inorganic polymer
particles via covalent bonds such as siloxanes, as well as the
solvation and subsequent ionic crosslinking of biopolymers.
1.2
In the sol–gel process, there are many different ways that a gel
can be formed. Sometimes, the same precursors can result in
very different structures with only small changes in conditions.
Generally, the gel state is simply defined as a non-fluid 3D
network that extends through a fluid phase. Gels were grouped
by Flory in 1974 into four types, including ordered, lamellar
gels (e.g. clays or surfactant mesophases), covalent polymer
networks, networks of physically aggregated polymers (e.g.
hydrogels formed via helical junctions) and finally disordered
particulate gels.5 However, for the purposes of sol–gel chemistry,
which is used for the preparation of inorganic solids, a more
useful classification of different gel types was given by Kakihana
in 1996.3 Fig. 1 outlines the five key types of gel that feature in
‘sol–gel’ chemistry. It is debatable whether metal complexes can
always be classed as gels since many of these actually form
viscous solutions or glassy solids rather than gels. However, the
fundamental goal of forming a homogeneous metal-containing
precursor is still applicable and the use of small molecules, often
generalized as the ‘citrate sol–gel’ method, is frequently cited in
the literature.
2. Gels via hydrolysis and
condensation
2.1
Zoe Schnepp is passionate about
green chemistry, both in her
research and the potential for
changing negative public perceptions of chemistry. With diverse
interests in nanotechnology,
catalysis and materials from
biomass, Zoe leads a growing
group in the School of Chemistry
at the University of Birmingham,
UK. Prior to this, she held
Postdoctoral Fellowships in the
International Center for Young
Z. Schnepp
Scientists at the National Institute
for Materials Science in Japan and the Max Planck Institute for
Colloids and Interfaces in Germany. She received her PhD from the
University of Bristol.
92 | Mater. Horiz., 2016, 3, 91--112
Types of gel
General introduction
The origin of sol–gel chemistry was the observation in the 19th
century that an alkoxide prepared from SiCl4 started to form a
gel when exposed to air.6 This was later found to be driven by
atmospheric moisture causing first hydrolysis of the silicon
alkoxide and then condensation. These processes have since
been widely studied and can be carefully tuned, for example
through acid or base catalysis, to form gels with very different
structures. Hydrolysis and condensation chemistry is limited by
the number of elements that readily form alkoxides and also
by the high reactivity of many of these compounds. Furthermore,
the very different rates of hydrolysis of different alkoxides can
lead to substantial phase separation and difficulty in synthesizing ternary or quaternary systems. However, this ‘traditional’
sol–gel chemistry is still one of the most widely used and studied
fields of materials chemistry.
Aside from precursor preparation, the sol–gel process can be
summarized in the following key steps:
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Fig. 1
Chart classifying 5 different types of gels that are relevant in sol–gel synthesis of materials.
(i) Synthesis of the ‘sol’ from hydrolysis and partial condensation of alkoxides.
(ii) Formation of the gel via polycondensation to form
metal–oxo–metal or metal–hydroxy–metal bonds.
(iii) Syneresis or ‘aging’ where condensation continues within
the gel network, often shrinking it and resulting in expulsion of
solvent.
(iv) Drying the gel either to form a dense ‘xerogel’ via
collapse of the porous network or an aerogel for example
through supercritical drying.
(v) Removal of surface M–OH groups through calcination at
high temperature up to 800 1C (if required).
2.2
Review
Precursors
Sol–gel chemistry originated with the hydrolysis and condensation of metal alkoxides, although it can also occur between
hydrated metal species. Most of the examples of alkoxide-based
sol–gel chemistry involve early transition group metals (e.g. Ti, Zr)
or early p-block elements (e.g. Al, Si), however there are many
other examples of elemental alkoxides. Metal alkoxides can be
prepared in a number of ways depending of the nature of the
metal. As with the original synthesis, metal chlorides can be
reacted with alcohols. Highly reducing metals, i.e. alkali metals
and lanthanides, can react directly with alcohols to produce the
corresponding alkoxide and hydrogen.12 A large number of
alkoxides, such as Ta(OR)5 (R = Me, Et, nBu), can be produced
via anodic dissolution of the metal in alcohol with an electroconductive additive such as LiBr.13 Following the successful
This journal is © The Royal Society of Chemistry 2016
synthesis of many mono-metallic alkoxides many groups have
also prepared bi-14 and ter-15 metallic alkoxides.
The suitability of the various alkoxides for sol–gel chemistry
and outcome of the reactions depends on several things. One
factor is how electronegativity differences between the oxygen
and metal affect the ionic character of the M–O bond, which
can be predicted using the partial charge model developed by
Livage et al.16 Another important effect is the electron donating/
withdrawing ability of the alkyl/aryl chain on the stability of the
alkoxy groups. Both of these factors ultimately direct gel structure by influencing the relative rates of hydrolysis and condensation and thus the degree of oligomerization or polymerization.
Finally, physical factors such and volatility and viscosity can
affect suitability of alkoxides for sol–gel chemistry.17
2.3
Hydrolysis and condensation of alkoxides
The key to mastering sol–gel chemistry of alkoxides requires
understanding of the central hydrolysis and condensation
reactions. These are strongly affected by process parameters
such as the nature of the R-group (e.g. inductive effects), the
ratio of water to alkoxide and the presence and concentration of
catalysts. The sol–gel chemistry of silica is typically driven by
either acid or base catalysts as the neutral reaction is very slow.
The structure of the resulting gel is significantly different
depending on the catalyst and this is due to the relative rates
of the hydrolysis and condensation reactions. Hydrolysis results
in the replacement of an alkoxy group with a hydroxyl with a
pentacoordinate transition state in both the acid (Scheme 1) and
base (Scheme 2) catalysed systems. Depending on the conditions
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Scheme 1
Acid catalysed hydrolysis of silicon alkoxides.
Scheme 2
Base catalysed hydrolysis of silicon alkoxides.
and the Si/H2O ratio, more than one alkoxy group may be
hydrolysed (eqn (1)). The rate of each hydrolysis step depends
on the stability of the transition state which in turn depends on
the relative electron withdrawing or donating power of –OH
versus –OR groups. The result is that successive hydrolysis steps
get progressively slower under acidic conditions and faster under
basic conditions.
Si(OR)4 + nH2O - Si(OR)4 n(OH)n + nROH
(1)
Condensation follows a similar pattern, being catalysed by
either acid (Scheme 3) or base (Scheme 4) and resulting in the
formation of siloxane bonds (or metaloxane bonds for other
metals). The progression of condensation depends on the
degree of hydrolysis that has already occurred as a silanol
group is required on at least one silicon centre. If hydrolysis
is complete before the first condensation step occurs, the
resulting product (OH)3Si–O–Si(OH)3 has 6 sites for subsequent
condensation steps. This is observed in basic conditions, where
hydrolysis steps get progressively faster. Multiple condensation
steps result in small, highly branched agglomerates in the ‘sol’
which eventually crosslink to form a colloidal gel. In acidic
conditions, where the first hydrolysis step is typically the
fastest, condensation begins before hydrolysis is complete.
Condensation often occurs on terminal silanols, resulting in
chain like structures in the sol and network-like gels. The
consequences for gel morphology are represented in Fig. 2.
In addition to acid and base catalysts, many other factors
can affect the rates of hydrolysis and condensation and thus the
Scheme 3
Acid catalysed condensation of silicon alkoxides.
Scheme 4
Base catalysed condensation of silicon alkoxides.
94 | Mater. Horiz., 2016, 3, 91--112
Fig. 2 Diagram showing how pH affects the growth and structure of a
gel; adapted with permission from ref. 2. Copyright (2004) American
Chemical Society.
structure of silica gels. One important influence is the presence
of solvents, either to enhance mixing (many silicon alkoxides
are immiscible with water) or direct interaction of solvent
molecules with the silicon centre. Water itself is important
and alkoxide : water ratio can be tuned to limit hydrolysis. Many
different silicon alkoxides exist and the inductive and steric
effects of the R group can impact on hydrolysis rates. It should
also be noted that molecular silicon chemistry is far more
diverse than simple tetraalkoxides and many compounds exist
with the general structure SiR(OR)3, SiR2(OR)2 or SiR3OR.
Finally, the presence of chelating agents such as acetylacetone
can also be a method to reduce hydrolysis and condensation
rates, although this becomes more important in the sol–gel
chemistry of other metals.
Other metal alkoxides in sol–gel chemistry can follow similar
reactions and pathways to silicon. However, most other metal
alkoxides are based on elements with substantially lower electronegativity than silicon, the most important being the early
transition metals such as titanium and zirconium. The partial
charge model considers electronegativity differences and can be
used to estimate stability and reactivity of alkoxides. For a fourcoordinate tetraethoxy complex (M(OEt)4), the partial charges for
Si, Ti and Zr can be calculated as +0.32, +0.63 and +0.74
respectively.2 Hydrolysis proceeds via nucleophilic attack by
either water or hydroxyl groups on the central metal and the
substantially higher rates of hydrolysis for Ti and Zr alkoxides
are reflected in their higher partial positive charge. An important
point to make here is that the variable reactivities of different
metal alkoxides can lead to problems in synthesizing ternary or
quaternary products due to phase separation during the condensation steps. The higher rate of hydrolysis means that many
transition metal alkoxides react violently with water to the extent
that most need special handling and storage. An important case
where careful handling is required is for titanium alkoxides.
Most of these will react vigorously with water to produce illdefined titanium–oxo/hydroxo precipitates. Unlike silicon,
where catalysts are added to enhance hydrolysis and condensation, titanium alkoxides therefore require additives to slow down
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Materials Horizons
the sol–gel reactions. Most of this chemistry centres on the use
of bidentate or multidentate ligands such as acetylacetone to
substitute for some of the alkoxide groups on the titanium,
although the precise solution structure of the metal alkoxides
and the formation of clusters are also important.18 The strength
of binding of the chelating ligand, as well as the ligand : alkoxide
ratio both impact the reactivity of the titanium precursor but
also have structural implications for the resulting gel. For
example if the number of OR groups is reduced, there are fewer
sites for hydrolysis resulting in less crosslinking in the final gel.
The chelating ligands may also have stereochemical effects by
directing hydrolysis and condensation to certain sites. These
effects are described in detail in an excellent review by Schubert.19
The control of metal sol–gel chemistry in this way has enabled the
synthesis of a wide range of crystalline and amorphous transition
metal oxide structures such as thin films20 or monodisperse
particles.21
2.4 Materials from sol–gel chemistry: processing,
post-processing and templating
In addition to the chemistry involved in forming a gel, there are
many ways to control the sol–gel process to introduce important chemical and structural features. One important factor is
how the sol or gel is physically treated because something
as simple as rate of evaporation during gelation can have a
substantial impact on gel structure. Heat treatment is also
important for drying gels as well as removing surface hydroxyl
groups, densifying the material to produce a ceramic monolith
or converting to a crystalline material.
There are many methods of processing a silica sol or gel and
many of these can be applied to gels produced from other
elements (ref. 4 and references therein). Processing sols or gels
can be as simple as fast stirring during hydrolysis and condensation to produce small particles, as exemplified by the
Stŏber synthesis.22 Another important feature of sol–gel processing is converting the solvent-filled gel into a dry solid. Simple
evaporation of solvent from a silica gel is possible, but the
movement of solvent through the gel subjects it to considerable
capillary forces resulting in collapse of the network. This can be
countered to some extent by aging the gel for a long time prior
to drying but some densification is unavoidable due to expulsion of the sol from within the gel body (syneresis). The
products of this uncontrolled drying (called xerogels) often
have a high surface area due to the large number of small
pores but without addition of structure-directing agents this
porosity is generally disordered. If a larger pore volume is
required, gels can be dried under supercritical conditions to
produce aerogels with up to 98% air (or other gas) by volume.23
In fact, sol–gel synthesis can also be carried out in supercritical
fluids to produce nanostructures of a wide range of materials. It
is also possible to achieve high levels of porosity through freeze
drying, this results in a cryogel, the porosity of which is usually
between a xerogel and aerogel.24 In terms of fibres and thin films
from alkoxide precursors, the most important factor is tuning the
water : alkoxide : solvent ratio.25 The resulting solution can then be
This journal is © The Royal Society of Chemistry 2016
Review
spun into fibres or spin/dip coated onto a surface to produce metal
oxide thin films.26
In addition to physical methods to control structure in
sol–gel chemistry, templates can also play an important role
in introducing both ordered and disordered porosity. The most
common additives have been ‘soft templates’, for example
amphiphiles, block copolymers, ionic liquids, biopolymers
and proteins.27 Alternatively, hard materials such as colloidal
particles, bacterial filaments or cellulose nanocrystals have
been employed.28 In some cases, alkoxides can be modified
to enhance interaction of the sol–gel precursors with a soft or a
hard template and produce ordering or porosity on multiple
length scales.29 In many of these examples, the template can
either be left in the oxide to produce an inorganic/organic
nanocomposite or removed by dissolution or calcination. In
addition, templating can be combined with functionalization
of the material, for example to generate porous silica that
incorporates molecular recognition sites.30
A particularly important field that uses amphiphiles for
‘templating’ sol–gel materials is evaporation-induced selfassembly (EISA). This can be used to introduce ordered mesoporosity into bulk or thin-film metal oxide materials.31 There
are many excellent reviews of the field and the mechanisms of
ordering in EISA.32 Briefly, the method involves a mixture of
sol–gel precursors such as water, ethanol and a metal alkoxide
or chloride, combined with amphiphiles such as cetyltrimethylammonium bromide (CTAB) or block copolymers. Rather than
simple direction of the sol–gel condensation within solution,
EISA relies on gradual evaporation of volatile species from the
mixture to form a mesophase. Inorganic material accumulates
around this liquid-crystal template, which results in wellordered mesostructuring in the resulting metal oxide.
It should be noted that while this section has focussed on
processing of alkoxide precursors, many of these methods can
also be applied to materials and techniques discussed later in
this review.
3. Small molecule ‘gels’
3.1
Hydrolysis reactions in aqueous solution
The scope of ‘traditional’ sol–gel chemistry is broad and it is
only possible here to give a flavour of the diverse reactions
possible with alkoxide chemistry. However, alkoxide-based sol–
gel chemistry has one significant limitation, namely that many
metal alkoxides either cannot be formed or are too unstable to
be used. A minor additional point is that if a metal alkoxide
is extremely sensitive to moisture it can often be employed in
sol–gel synthesis with careful handling, but water-soluble
structure directing agents or templates may not be compatible.
Given the power of sol–gel chemistry, many alternative methods
have been developed that can employ aqueous metal salts rather
than alkoxides. The chemistry is very different but the goal is the
same: the controlled formation of metal oxide or other ceramic
structures from solution-phase precursors. The first main strategy
involves the use of small molecules (often chelating agents) to
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Materials Horizons
modify the aqueous hydrolysis chemistry of metal ions. For an
aqueous solution of metal ions of charge z+, water molecules
coordinate to the metal via electrons in their bonding orbitals. In
the case of transition metals, this density is transferred into
empty d-orbitals. The result is a weakening of the O–H bonds
of bound water molecules and, depending on the pH, deprotonation or hydrolysis (eqn (2)). For highly reactive metals, the high
rate of hydrolysis may necessitate the control of water content and
the use of nonaqueous solvents. For less reactive metals, where an
aqueous solution is readily achieved, the extent of hydrolysis can
be controlled easily with pH. Basic conditions will force the
equilibrium to the right and favour the formation of oxo ligands,
whereas acidic conditions can favour hydroxo ligands or even
prevent hydrolysis. For some metals, such as Fe3+, hydrolysis can
result in extended polymer-like precipitates containing oxo and
hydroxo bridges. In addition to pH, highly charged metal cations
tend to weaken the O–H bond in bound water and favour
hydrolysis. This relationship between charge, pH and hydrolysis
equilibria is shown in Fig. 3a.
[M(OH2)]z+ " [M–OH](z
1)+
+ H+ " [MQO](z
2)+
+ 2H+
(2)
For many metal salts the aqueous solution is stable. However, while the solution is homogenous, it does not resemble
the gels of sol–gel chemistry. The formation of most metal
oxides (and other ceramics) requires heat treatment and simply
drying a metal salt solution will result either in precipitation of
the original metal salt, or amorphous oxides/hydroxides. The
result is typically large crystals or aggregates, certainly not the
controlled formation of porous structures or regular particles of
sol–gel methods. To avoid this, many small molecules have
been employed to form stable aqueous metal complexes and
structures that more closely resemble ‘gels’. Many of these
small molecules are chelating agents and the main function
is to change the hydrolysis equilibria of dissolved metals. For
example, the addition of EDTA (ethylenediaminetetraacetic
acid, Fig. 3b) to aqueous iron can significantly reduce the
equilibrium constant of hydrolysis from Kh = 10 3 (eqn (3))33
to Kh = 10 7.5 (eqn (4)).34 By making hydrolysis considerably less
favourable, the removal of solvent from many metal–chelate
solutions results in homogeneous glassy solids or resins rather
than precipitates. These can be heat treated to form powders or
nanostructures of a wide range of binary, ternary and ternary
metal oxides as well as metal nitrides or carbides.
[Fe(H2O)6]3+ + H2O " [Fe(OH)(H2O)5]2 + H3O+
(3)
[Fe(H2O)2EDTA] + H2O " [Fe(OH)(H2O)EDTA]2 + H3O+
(4)
In this section, we will consider some of the most important
examples of small molecule sol–gel chemistry in detail. This
field has a huge scope, with multiple factors that can be tuned such
as pH, concentration and nature of complexing ligand, temperature
of gelation, rate and final temperature of calcination.35
3.2
Citrate
One of the most common small organic molecules used in
sol–gel chemistry is citric acid. Citric acid is a weak triprotic
acid (Fig. 3c) with three carboxylic acid moieties that are able to
dissociate (eqn (5)–(7)).36 While being readily available and
cheap it is also an effective chelating agent. In a typical
synthesis, aqueous metal salts (e.g. nitrates) are mixed with
citric acid and the resulting solution heated to form a viscous
solution or gel. Some reports describe the addition of bases
such as ammonia or ethylene diamine to modify the pH and
enhance cation binding to the citrate. The homogeneity and
stability of metal citrate solutions can thus depend strongly on
pH (Fig. 3d).37 Tuning of pH appears to be particularly important
in systems with several different metals, in order to optimize the
formation of stable metal citrate species and prevent precipitation of individual hydroxides.38,39
H3Cit " H2Cit + H+
Ka1 = 7.10 10
H2Cit " HCit2 + H+
Ka2 = 1.68 10
HCit2 " Cit3 + H+
Ka1 = 6.40 10
4
5
6
(5)
(6)
(7)
The citric acid sol–gel method (also referred to as the citrate
sol–gel method) is normally used for the synthesis of metal
Fig. 3 (a) Relationship between charge, pH, and hydrolysis equilibrium of cations, modified with permission from ref. 2. Structures of (b) EDTA and
(c) citric acid. (d) Ion speciation graph for citric acid plotted using the program HSS.
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Materials Horizons
oxide powders. Conversion of the ‘gel’ to a metal oxide is simply
achieved by pyrolysis in air, with the maximum temperature
depending on the specific system. The method has been used
to synthesize binary, ternary and quaternary metal oxides in
both crystalline and amorphous forms. The key advantage of
this method, as with more traditional sol–gel chemistry, is the
homogeneity of the starting material. As the metal–citrate ‘gels’
are heated, the organic component undergoes combustion
at B300–400 1C, depending on the metal counterion and
presence of additives. The presence of the organic matrix
during the first stages of synthesis can ensure that when
nucleation occurs, the sites are evenly dispersed and numerous, ensuring a small crystallite size. In the case of ternary or
quaternary systems, the other purpose of the matrix is to ensure
that the different metals remain mixed on an atomic scale.
Even in systems where the desired compound may not crystallise until 4700 1C, long after the organic component has been
combusted, the homogeneity of the precursor gel can ensure
that the system remains amorphous until the final product
begins to nucleate. This is important in ternary and quaternary
metal oxides such as doped yttrium aluminium garnets (e.g.
Y3Al5O12, YAG), where the nucleation and growth of intermediate
phases can disrupt the homogeneity of the system and result in
impurities or particle size irregularity in the final product.40
Another impact of the homogeneity of citrate sol–gel precursors
is on reaction temperature since the final crystalline metal oxide
may be formed at considerably lower temperatures than powder
solid-state methods where mass transport between grains limits
the reaction.41
Although most reports describe an air atmosphere, it should
be noted that inert atmospheres can also be used to produce
ceramic/carbon composites where the citrate provides the
carbon source. One example is the synthesis of carbon/LiFePO4,
where the reducing conditions also have the effect of preserving
the ferrous oxidation state of the iron precursor. In this case, the
resulting material is a composite of small particles of the LiFePO4
(an important cathode material for lithium ion batteries) dispersed
in a carbon matrix that enhances electronic conductivity.42 In
addition to oxide/carbon composites, inert atmospheres can be
used to transform citrate precursors into reduced materials
such as metal carbides43 or metal borides.44
The majority of work reported on the citrate sol–gel method
uses metal nitrate precursors. Rather than simply being a
convenient source of aqueous metal ions, the nitrate counterion
plays an important role in citrate sol–gel chemistry. Thermogravimetric analysis of various metal nitrate/citrate combinations reveal a very sharp mass loss step associated with an
exothermic peak in the differential thermal analysis (DTA) trace
(Fig. 4a).45 The mass loss typically occurs around 200 1C and is
associated with a rapid, self-propagating combustion where the
nitrate acts as the oxidant and the citrate as the organic fuel
(Fig. 4b–e).46 In some systems, the combustion can be triggered
by ignition of the sample at room temperature to form loose
powder. These may require further heat treatment to achieve the
desired crystalline phase but in many cases enough heat is
generated during the combustion process.47 The crystallinity
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Review
Fig. 4 (a) TGA/DTA trace for a mixture of citrate with barium and iron
nitrates showing a sharp combustion at B200 1C. (b–e) Images from a
video of a combustion synthesis showing rapid progression of the reaction. (f) TEM image of a sample of a NiCuZn ferrite synthesized from a
citrate/nitrate combustion method. Figures modified with permission from
ref. 45, 46 and 48 respectively.
and morphology of powders prepared by citrate auto-combustion
can depend on pH. For example, in the synthesis of NiCuZn ferrite
from nitrate/citrate precursors, an increase of pH in the precursor
solution resulted in oxide products with a more open and porous
network structure (Fig. 4f).48 In fact, many citrate combustion
syntheses result in ‘sponge-like’ products due to the large volume
of gases evolved during the reaction of nitrate with the organic
component.49 In this particular example, the pH was modified via
addition of ammonia, resulting in a build-up of NH4NO3. The
exothermic peak for high pH (46) was sharp and the product
more porous due to the decomposition of NH4NO3 into NOx and
O2 which accelerates combustion.
3.3
Other aqueous small molecule gelators
Given the success and wide application of citric acid in sol–gel
processing, it is perhaps not surprising that other mono- and
di-carboxylic acids have also been employed. These include
tartaric acid,50 glycolic acid51 and oxalic acid.52 As with citric
acid, the pH of the system is important to optimize binding of
the carboxylate to the metal and many of these examples use
ammonia or other bases as an additive.53 Interestingly, the
large range of dicarboxylic acids offers a way to control the
decomposition profile of a nitrate/carboxylate gel in combustion synthesis. Linear aliphatic dicarboxylic acids from oxalic
(HO2CCO2H) to sebacic (HO2C(CH2)8CO2H) acid were used to
synthesize LiNi0.8Co0.2O2. The gels showed a linear increase in
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Materials Horizons
decomposition temperature with increasing length of the dicarboxylic acid.54 Given that persistence of the organic matrix may
have some influence on particle size in sol–gel synthesis, this
offers a useful way to study aqueous sol–gel processes across a
wide range of systems.
Many other small molecules have been combined with metal
nitrates as a fuel/oxidant combination for oxide powder synthesis.55 Glycine is a common chelator and ‘fuel’ and has a lower
ignition temperature of combustion than citrate.56 This can be
advantageous. Glycine–nitrate mixtures burn quickly, releasing
gases and forming ‘fluffy’ powders.57 However, the reaction is
highly exothermic and in some cases explosive, which may
preclude large scale-up of the process.58 A modification to the
glycine–nitrate method that helps to mitigate the violent reaction and gas evolution is to soak the precursors into cellulose
fibres.59 The cellulose fibres act as a micro-reactor for the sol–
gel process and also help to ensure a very small particle size
(15–20 nm) of the resulting La0.6Sr0.4Co0.2Fe0.8O3 by maintaining spatial separation of the nucleation points and growing
crystallites. Other notable small molecule gelators to be used in
sol–gel synthesis are ethylenediaminetetraacetic acid (EDTA),60
glucose61 and amino acids such as glutamine and histidine.62
3.4
sol–gel chemistry.65 However, the author performed a detailed
analysis of the decomposition products during synthesis which is
instructive when considering later ‘gel’ examples. It should also be
noted that gallium and indium have very low melting points and
most of the reactions in this paper would occur above this,
therefore it is certainly not a ‘normal’ solid-state process. Urea
undergoes endothermic decomposition to form numerous compounds, for example biuret and triuret via initial condensation of
two and three urea molecules respectively. Subsequent condensation and dehydration reactions result in release of NH3 and H2O
and formation of compounds including dicyandiamide, cyanuric
acid, melamine and melem (a tri-s-triazine). In the reaction of
gallium with urea,66 the author concludes that gallium reacts with
some of the urea decomposition products to generate polymeric
intermediates. These undergo further decomposition to produce
GaN although pure GaN could only be isolated if the process was
carried out in an ammonia atmosphere.
In later work, it was demonstrated that phase-pure metal
nitrides could be synthesized under an inert atmosphere (e.g. N2
or Ar) directly from ‘gel-like’ precursors.67 In general, metal
chlorides such as MoCl5 or WCl4 are dissolved in ethanol, releasing HCl gas and producing and ethanolic solution of the metal
Epoxide sol–gel
The chemistry involved in the epoxide ‘sol–gel’ method is
somewhat different from the other examples in this section.
However, the overall strategy still involves the addition of a
small molecule to influence hydrolysis of a dissolved metal salt
and so it is useful to discuss it here. In general, the method
involves dissolving hydrated metal salts in ethanol (or other
polar protic solvents). Propylene oxide is then added to drive
formation of a gel. The nature of the metal counterion (e.g. Cl ,
NO3 ) the solvent and the amount of water and propylene oxide
are all critical to the appearance and also rate of formation of
the gel.63 Unlike the chelating agents in the previous examples,
the role of propylene oxide in this approach is as a proton
scavenger, driving the formation of metal–oxo bonds. For example,
in the synthesis of iron oxide using propylene oxide, the epoxide
additive is believed to drive firstly the hydrolysis of hydrated iron
species (eqn (8)), which then leads to condensation to form iron
oxo (Fe–O–Fe) bonds (eqn (9)). In this sense, the epoxide sol–gel
method bears some resemblance to silica sol–gel chemistry.
[Fe(H2O)6]3+ + H2O " [Fe(OH)(H2O)5]2+ + H3O+
2+
2Fe(OH)(H2O)5]
3.5
" [(H2O)5FeOFe(H2O)5]
4+
+ H 2O
(8)
(9)
Urea
Like the epoxide method, the use of urea in sol–gel chemistry is
somewhat different to many examples of ‘small molecule
gelators’ in that water is generally not used as the solvent.
There are examples of urea being used in a standard fuel/
nitrate sol–gel combustion synthesis to form metal oxides.64
However, the most recent urea chemistry has involved the
synthesis of metal nitrides and carbides. The earliest examples
of using urea to make nitrides involved heating metals (e.g.
gallium or indium) with urea and so cannot be considered as
98 | Mater. Horiz., 2016, 3, 91--112
Fig. 5 (a) TGA/DTG of tantalum/urea mixtures both with (black) and
without (red) calcium showing the later onset of decomposition with
calcium. (b) IR spectra of the same samples showing a shift and reduction
in intensity of the CQO stretch of urea when calcium is added. Modified
with permission from ref. 70.
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alkoxides. Solid urea is then added and the resulting solution
dried to give a glassy solid. Heating these solids to 800 1C under
nitrogen results in molybdenum or tungsten nitrides or carbides
depending on the metal : urea ratio. The products are comprised
of nanoparticles (diameter 4–20 nm) embedded in amorphous
carbon. The method was later extended to synthesize nanoparticles of nitrides and carbides of Ga, Ti, Nb and V.
The formation of nanoparticles is a significant step, given the
importance of metal carbides and nitrides in catalysis.68 In the
case of iron carbide (Fe3C), the ability to form small nanoparticles
(5–10 nm) using urea meant that the product was superparamagnetic (each nanoparticle is a single magnetic domain).69
Calcium can assist the urea sol–gel synthesis. This has proven
effective in isolating TaON and Ta3N5, both important semiconductor photocatalysts with a smaller bandgap than the corresponding oxide. Previous methods result in the formation of
complex mixtures of oxynitrides and nitrides but the use of
calcium/urea precursors enabled the tunable formation of single
phases by simply changing the Ta/urea ratio.70 The authors use
infrared spectroscopy and thermogravimetric analysis to propose a
mechanism that involves Ca2+ binding to urea via the carbonyl
oxygen, weakening the CQO bond and strengthening the C–N
bonds (Fig. 5). The result is that the urea decomposition is delayed,
slowing the release of NH3 and creating a more homogeneous and
persistent atmosphere within the evolving system. Importantly, the
use of calcium also produces a much more homogeneous product
with smaller crystallites and higher surface area.
4. Pechini method
4.1
Chemistry of the Pechini method
The Pechini method, named after the author of the original
patent,9 builds on the principles of sol–gel chemistry involving
small molecule chelating ligands in that the initial step is to
form a homogeneous solution of metal/citrate complexes.
Scheme 5 The transesterification reaction that occurs between citric
acid and ethylene glycol in the Pechini process.
Fig. 6
Review
However, the Pechini method takes this further to convert the
mixture into a covalent polymer network to entrap the metal
ions. The reasoning for the method was to delay the thermal
decomposition of the organic matrix in order to afford more
control over the growing ceramic product. The key reaction
used in Pechini synthesis is transesterification between citrate
and ethylene glycol (Scheme 5).71 In a typical synthesis, a metal
salt is dissolved in water with citric acid and ethylene glycol to
form a homogeneous precursor solution containing metal–citrate
chelate complexes. This solution is heated to initiate polyesterification between the citrate and ethylene glycol, forming an
extended covalent network. Fig. 6 shows a proposed schematic
of this process. Following formation of the polymer network,
the material is heated in a furnace to combust the organic
matrix and form the ceramic product. One of the most significant advantages of the Pechini method is the ability to form
a polymeric precursor where two or more metals may be
dispersed homogeneously throughout the network.
4.2
Materials from the Pechini method
Many authors have employed the Pechini method to synthesize
metal oxides by combining metal salts with ethylene glycol and
citric acid and treating the resulting resin in a furnace in air.
Multiple variations have been reported in order to optimize the
method to a particular system. As described in the citrate
sol–gel section above, the binding of citrate to metal ions
depends on the pH of the solution, with low pH resulting in
protonation of the citrate and high pH risking precipitation of
metal hydroxides. The control of pH in the citrate sol–gel
method is thus very important in controlling homogeneity in
the gel and particle size in the final product. Similarly, many
examples of the Pechini method report optimizing the pH in
the initial metal–citrate solution using ammonia, ammonium
hydroxide or other bases.72 One method of adjusting citrate
binding in the Pechini method was proposed by Abreu Jr. et al.
Rather than using a base such as ammonium hydroxide, the
authors use urea, which gradually decomposes in the system to
release ammonia and result in a more controlled rise in pH.73
The effect on the product is striking. The urea-modified method
produces much smaller crystallites of Pb(Zr0.52Ti0.48)O3, although
it is unclear whether this is exclusively due to the pH control
or also the increased spatial separation of the nuclei by urea
condensation products such as melamine.
In addition to metal oxides, the Pechini method has been
used to synthesize transition metal carbides. These have been
Schematic of the Pechini method of making metal/organic gels.
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employed for many years for their mechanical properties but
more recently have attracted interest as catalysts. The method
is essentially the same as for oxides, but the precursor gel or
resin must be heated in an inert or reducing atmosphere
instead of air. For example, Ni6Mo6C can be synthesized by
heating metal acetates, citric acid and ethylene glycol under
hydrogen to 600–900 1C The product structure depends strongly
on the maximum temperature, being mesoporous with a surface area of 96 m2 g 1 when heated to 800 1C but sintering to a
surface area of only 5 m2 g 1 after heating to 900 1C.74
Most examples of Pechini synthesis of ceramics result in
powders of agglomerated crystallites. However, if metal nitrates
are used in the initial mixture, it is possible to generate foams
through release of nitrous oxides during the reaction, analogous
to many examples of combustion synthesis.75 Alternatively, it is
possible to fill a porous template with the mixture of metal salt,
ethylene glycol (EG) and citric acid (CA) and heat to polymerise the
network inside the template. This has been used in several cases
to produce polycrystalline wires using anodised alumina templates.76 Pechini precursors can also be used to create thin films.
This is possible with alkoxide-based sol–gel precursors. However,
the aqueous nature of the Pechini method enables the production
of films from a wider range of elements as water-soluble salts
are more readily available and easier to work with than metal
alkoxides. For example, Eu-doped Lu2O3 films can be prepared by
mixing LuCl3, water, ethanol, citric acid, polyethylene glycol and
Eu(NO3)3 and dip coating the resulting solution onto silicon
wafers.77 The ability to create films of ceramics is important in
various applications, particularly those involving light absorption
and emission such as displays.
Mesoporous oxides have also been produced in a similar way
by templating with colloidal crystals and this latter example has
Materials Horizons
given some remarkable insights into the mechanism of how the
Pechini precursors decompose to form a ceramic. By changing
various experimental parameters, Rudisill et al. showed that the
structure of CeO2 and Mg/Ca/Sr-doped ceria could be tuned to
form either mesoporous microspheres or a bicontinuous mesoporous network.78 The authors showed that the structure was
dictated by phase separation in the early stages of the synthesis
i.e. during the polyesterification process, analogous to the
polymerization-induced phase separation that can be achieved
in sol–gel synthesis of silica from alkoxides.79 This resulted in
either a nucleation mechanism or the development of a bicontinuous structure through spinodal decomposition (Fig. 7).
A detailed investigation showed that many factors could affect
the final structure, including the nature of the alkaline earth
metal in the ternary systems (e.g. Ce0.5Mg0.5O1.5 vs. Ce0.5Ca0.5O1.5),
the metal : CA ratio and the amount of EG. The confinement effect
of the template was critical for the formation of the unusual
structures. This is due to both the physical effect of confining the
precursors as well as electrostatic interactions of the soluble
precursors with the charged surface of the PMMA (polymethylmethacrylate) spheres of the colloidal crystal.
4.3
Modifications to the Pechini method
Since the early examples of Pechini synthesis, there have been
many developments to enhance the range of materials and
structures that can be achieved via this process. Most of these
have centred on replacing the citric acid with other di-, tri- or
tetra-carboxylic acids and/or the ethylene glycol with other
polyols. Early examples focussed on the substitution of citric
acid with chelating agents that have a higher decomposition
temperature, such as EDTA (ethylenediaminetetraacetic acid).
In the synthesis of the YBCO superconductor YBa2Cu3O7 x, this
Fig. 7 (a) SEM images of samples prepared at two EG : CA : TMI (ethylene glycol to citric acid to total metal ion) ratios, (b) schematic showing how
EG : CA : TMI ratio affects structure and (c) schematic of nucleation and spinodal decomposition mechanisms. Modified with permission from ref. 78.
Copyright (2012) ACS.
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was shown to aid the formation of a phase pure product. The
synthesis of quaternary oxides like YBCO superconductors is
often hampered by the uncontrolled nucleation and growth
of intermediate phases that impede the reaction and lead
to impurities in the final product. Replacement of citric acid
with tartaric acid or EDTA was shown to limit the formation of
unwanted barium carbonate. This was proposed to be due to
the later onset of thermal decomposition of the EDTA/ethylene
glycol polymer i.e. a longer period of homogeneity at the start of
the heating process. Interestingly, the EDTA system produced
larger particles than citric or tartaric acids, which could indicate that the structure and degree of branching of the polyester
influences particle size.80
Further insight into the specific effect of different precursors
in Pechini synthesis came from some work by Rudisill et al.81
These authors previously discussed the synthesis of mesoporous microspheres or a bicontinuous mesoporous network
of doped cerium oxides from Pechini precursors in a PMMA
(polymethylmethacrylate) opal template.78 In a more recent
publication, the authors investigated the substitution of citric
acid (tricarboxylic acid) and ethylene glycol (diol) with malic
acid (dicarboxylic acid) and glycerin (triol) respectively (Fig. 8a
and b). Phase separation in the PMMA-templated systems is
dependent on the polyesterification process and, importantly,
the degree of polymerization. Changing the ratio of carboxylic
acid : hydroxyl moieties therefore enables a study of functional
group balance without changing the overall metal : organic ratio
in the system. In other words, the number of reactive groups
Review
available for polyesterification can be varied in a controlled way.
The result was a dramatic change in sample morphology (microspheres vs. bicontinuous network) with a simple change in the
carboxyl : hydroxyl ratio i.e. extent of polyesterification (Fig. 8c–f)
The authors postulate that a low level of polyesterification would
lead to small and relatively soluble oligomers that can evenly fill
the space in the PMMA template, minimizing polymerizationinduced phase separation and leading to a continuous structure.
A high level of polymerization and a correspondingly high
molecular weight of the polyester would lead to polymer-rich
regions in the aqueous solution. The difference in polarity
between the solvent-rich and polymer-rich regions drives the
formation of microspheres as the system attempts to minimize
interfacial energy.
5. Polymers
The purpose of the Pechini synthesis is to synthesize a polymer
in situ around metal ions and thus ensure the metals are mixed
and stabilized homogeneously within a covalent matrix or ‘gel’.
Therefore, it seems logical that the next step in the evolution of
sol–gel chemistry involved the direct combination of polymers
with metal salts to form sol–gel precursors. Polymer chemistry
is diverse and many polymers form well-defined structures in
solution, some of these with quite long-range order. Many
polymers also exhibit strong interactions with metal ions.
Despite the superficial similarity between polymer sol–gel
methods and Pechini synthesis, there are specific advantages
to using polymers. In particular, the ability of some polymers to
form ordered superstructures can be used to control morphology in ceramic synthesis. However, polymers can bring some
drawbacks, as will be discussed in this section.
5.1
Fig. 8 Structures of (a) malic acid and (b) glycerine. SEM images of
samples of Ce0.5Mg0.5O1.5 prepared at a 2 : 1 : 1 molar ratio of EG : CA : TMI
(ethylene glycol to citric acid to total metal ions) in a PMMA opal template
showing (c) no reagent substitutions, (d) substitution of glycerine for EG,
(e) substitution of malic acid for CA, and (f) substitution of both glycerine
and malic acid for EG and CA, respectively (c–f modified with permission
from ref. 81). Copyright (2015) American Chemical Society.
This journal is © The Royal Society of Chemistry 2016
Synthetic polymers
Many reports using synthetic polymers in sol–gel chemistry
focus on polyvinyl alcohol (PVA). The method is simple: aqueous
metal salts (e.g. metal nitrates) are mixed with polyvinyl alcohol
to form a homogeneous precursor that is heated at moderate
temperatures (B80 1C) to form a gel.82 This gel is typically dried
and then heated in a furnace to obtain the required metal oxide
ceramic. In some cases, the dried gel is powdered before the
final heat treatment. The presence of the polymer ensures that
particle size is kept very small (B25 nm), even in the case of
complex quaternary oxides such as YBCO (YBa2Cu3O7 x).83 This
control over ceramic crystallite growth is believed to be due to
complexation of the metal ions by hydroxyl substituents on the
PVA. This was postulated by Liu et al. who showed that particle
size homogeneity and phase purity of BiFeO3 was dependent on
Mx+/–OH ratio in the PVA precursor.84 These authors also
demonstrated the presence of carboxylate moieties, thought to
be caused by oxidation of the PVA by nitrates in the system and
this could conceivably aid metal binding. As with small-molecule
gels, the metal nitrate precursors in a PVA sol–gel method can be
exploited to initiate self-propagating combustion. This can be
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enhanced by the addition of supplementary ‘fuels’ to the PVA/
metal nitrate mixture, such as urea, hexamine or citric acid.85
In addition to PVA, several other synthetic polymers have
been employed in sol–gel synthesis of ceramics such as polyethylene glycol (PEG)86 or polyvinylpyrrolidone (PVP).87 As with
PVA, most of the methods involve dissolving metal salts such as
nitrates or acetates with the polymer in a solvent such as water
and heating to form a gel. The main benefit of the polymer
is the same in all cases. Functional groups on the polymer
(e.g. imide on PVP) bind to the dissolved metal ions to form a
homogeneous gel, which constrains particle nucleation and
growth, generating a nanoparticulate ceramic product. The
benefit of different polymers comes primarily from their physical
properties. For example, PVA and PVP have both been employed
in sol–gel precursors that can be spin-coated onto a substrate to
form ceramic thin films such as PLZT (Pb0.92La0.08Zr0.52Ti0.48O3).88
A sol–gel precursor of polyacrylonitrile (PAN) and poly(urea-coformaldehyde) methylated resin (PUF) with chromium chloride in
N,N-dimethylformamide (DMF) can be electrospun to give thin
fibres. Heating these to 1000 1C in a nitrogen atmosphere resulted
in carbon fibres that contain nanoparticles of chromium nitride/
carbide.89
5.2
Biopolymers
As highlighted in the section on synthetic polymers, the main
requirements for polymers in sol–gel chemistry is that they are
soluble in at least one solvent (preferably water) and also
contain functional groups that can bind to metal ions. In this
sense, the polymers that are available in nature are the ideal
resource as many of them dissolve readily in hot water.
Although this chemistry is quite different from the examples
of silicon alkoxides, biopolymers in solution still fall within
standard definitions of a ‘sol’ in that their chains extend for
1 nm–1 mm. The sol–gel transition can then be triggered by
cooling the solution and/or by addition of metal ions since
many biopolymers form complex, organized gels in water
through binding with metal ions. As will become clear in this
section, the extended, organized gel structures formed by
biopolymers can be exploited in sol–gel chemistry to direct
ceramic formation in a unique way.
The term ‘biopolymer’ is used in several situations, but for
the purposes of this review, we will consider a biopolymer to be
a macromolecule that has been extracted from biomass, i.e. a
polymer that has been synthesized by a living organism. With
such wide range of biological sources to choose from, different
biopolymers have very different properties and so some care is
needed when selecting a biopolymer for sol–gel chemistry. This
can be a useful handle for tuning material properties. The
diverse applications of biopolymers in templating90 and nanofabrication91 have previously been reviewed. However, in this
paper, we will discuss the specific use of biopolymers in sol–gel
chemistry, highlighting their advantages and disadvantages.
5.2.1 Common structures. Before discussing specific biopolymers, there are some common molecular features that are useful
to highlight. In general, biopolymer sol–gel chemistry has focussed
on polysaccharides and polypeptides. Polysaccharides are chains
102 | Mater. Horiz., 2016, 3, 91--112
Materials Horizons
Fig. 9
(a) Alpha and (b) beta glycosidic linkages.
of monosaccharide units typically featuring 6-membered pyranose
rings connected by glycoside bonds. These can form between
different carbon atoms on the rings, the most common linkage
being between carbons 1 and 4 to generate a linear polymer,
denoted 1 - 4. There are two possible configurations, a and b,
depending on the configuration of the glycoside bond as shown in
Fig. 9. As an example, cellulose is a linear polymer of b-(1 - 4)-Dglucopyranose units. Another distinguishing feature of polysaccharide biopolymers is the variety of chemistry in the functional
side-groups on the polymers. These include hydroxyls (agar, starch,
cellulose), carboxyls (pectin, alginate), amide/amines (chitin/
chitosan) and sulfate (carrageenan). In contrast to the polysaccharides, polypeptides are formed from covalently linked
amino acids. Given the diverse chemistry of amino acids, the
structural and chemical properties of polypeptides depend on
the amino acid ratios and sequencing. This is described in
more detail below. It is important to highlight at this stage that
many biomass sources consist of mixtures of different biopolymers, often in complex superstructures such as crystalline
fibres and sometimes in a composite with inorganic compounds such as calcium carbonate. Other types of biopolymer
exist, but extracted polysaccharides and polypeptides are the
most common and the most widely used in sol–gel chemistry.
Given the vast array of biopolymers available it is not possible
to discuss them all individually so we have focussed on five that
have been used most commonly.
5.2.2 Starch. Used as an energy store in most green plants,
starch is made of a mixture of amylose and amylopectin in a
ratio of approximately 1 : 4; although this can vary between
species. Both amylose and amylopectin are homopolymers of
a-D-glucose. Amylose contains a(1 - 4) glycosidic links and as a
result is linear and can form helices. Amylopectin is a complex,
highly branched polymer built from a(1 - 4) linked glucose
units with non-random a(1 - 6) links approximately every
30 units, providing branch points. The solubility of starch depends
mainly on the ratio of amylose to amylopectin but generally starch
is insoluble in cold water. The hydroxyl substituents on starch
biopolymers are readily modified, which can affect the physical
properties and, importantly, metal binding.92
Starch can be used to produce nanoparticles of various
oxides such as the pigment CoxZn1 xAl2O4 by simply heating
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Fig. 10 Distribution of functional groups on starch-derived carbons prepared at different temperatures (grey scale to indicate relative amounts
where black is highest). Modified with permission from ref. 95.
a mixture of aqueous metal salts with starch to form a gel,
followed by calcination in air.93 The starch behaves as a
chelating agent and the long chains of the molecule restrict
growth and sintering of nanoparticles. In the synthesis of many
materials, for example luminescent doped YVO4, nanoparticles
are important and the high level of control offered by the starch
synthesis brings advantages over many other synthesis techniques.94 In addition to metal oxides, starch gels can be heated in
an inert atmosphere or vacuum to produce carbons with a wide
range of surface chemistries, depending on the temperature
(Fig. 10), i.e. the degree of decomposition of the polysaccharide.95 These surface functionalities can be used to tune the
properties of the carbon, particularly in separation or catalysis
and can also be modified e.g. by post-treatment with H2SO4 to
give sulfonic acid groups and a solid acid catalyst.96 The
structure of these carbons depends on firstly drying the starch
gel in a controlled way (e.g. supercritical drying or solvent
exchange) to maintain the open gel network and produces
homogeneous mesoporous solids. It is also possible to achieve
metal–carbon composites, either by decorating starch-derived
carbons with metal nanoparticles97 or through calcination of
metal/starch gel precursors in inert atmosphere.98 A final
alternative is to directly produce porous metal carbides such
as SiC by using the starch as both the gel precursor and the
carbon source.99
5.2.3 Dextran. Dextran is a complex, highly-branched polysaccharide that can have molecular weights in excess of
1 000 000 Da. A typical structure of this biopolymer is a glucan
formed primarily of a(1 - 6) glycosidic repeating units with
side chains linked to the backbone via a(1 - 2), a(1 - 3) and
a(1 - 4) glycosidic bonds (Fig. 11).100 Dextran is produced by
enzymatic action of bacteria such as Leuconostoc mesenteroides
on sucrose and as a result the main side groups are hydroxyls,
although the polymer also contains reductive aldehyde substituents. Dextran is soluble in water over a large range of
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Review
Fig. 11 Structure of Dextran showing the different glycosidic bonds.
concentrations with the ability to bind to metals and potentially
reduce them in solution to form metallic nano- and microstructures. Dextran is particularly useful in this type of synthesis as it is highly soluble, stable and biocompatible, which is
important in biomedical applications. An advantage of dextran
in sol–gel chemistry is that it is readily modified, for example to
produce anionic dextran sulfate or carboxymethyl–dextran
where metal binding may be enhanced.
A comprehensive study of the use of dextran in materials
synthesis produced sponge-like structures of a range of compounds including metals and metal oxides.101 In general,
aqueous metal salts were mixed with dextran to produce
viscous liquids that could be shaped into monoliths or drawn
out into macroscopic wires. Drying to remove water and then
heating in air to 800 1C removed the dextran to produce
reticular structures, for example gold, silver or copper oxide
as well as composite materials. In the case of gold and silver,
the dextran reduced the metal ions during the initial drying
process although presumably some sintering occurred during
the final heating process. Similarly, sponges of YBa2Cu3O7 d
can be synthesized by dissolving dextran in an aqueous
solution of the relevant metal nitrates to form a paste that is
dried and then heated in air to 920 1C.102 This synthesis was
also extended to use carboxylated crosslinked dextran beads
(CM-Sephadexs) as a precursor rather than soluble dextran.
The resulting YBa2Cu3O7 d maintains the spherical shape of
the dextran precursor, with the microstructure being composed
of agglomerated nanoparticles.103
5.2.4 Chitin/chitosan. Chitin is one of the most abundant
polysaccharides in the biosphere, second only to cellulose. It is
the structural biopolymer of the hard shells and exoskeletons of
crustaceans and insects and is also found in fungi. In crustacean
shells, chitin is typically found in hierarchical fibrils and fibres
that are surrounded by crystals of calcium carbonate that enhance
hardness of the composite. Chitin is a linear polysaccharide, with
b(1 - 4) glycosidic links between the N-acetylglucosamine sub
units (Fig. 12a). The acetylamine substituents on the chitin
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polysaccharide lead to strong hydrogen bonding between the
chains and means that chitin will only dissolve in strong base
or some ionic liquids.104 For the purposes of sol–gel chemistry,
chitin can be modified through deacetylation to produce chitosan,
which dissolves in dilute aqueous acid.
Chitosan has the ability to sequester metal cations along the
length of its chain105 and can thus form homogeneous gels with
various metal ions. In the synthesis of type-II superconductors,
crystallite polydispersity and grain boundary misalignment can
reduce the critical current density and so control over morphology is important. Producing useful shapes such as wires is also
a step forward generally in the synthesis of electronic quaternary
oxide ceramics. Hall showed in 2006 that the binding of Y3+, Ba2+
and Cu2+ ions by chitosan in an aqueous gel could be used to
synthesize nanowires of the Y124 superconductor (YBa2Cu4O8).11
The biopolymer matrix chelates the metal precursors and ensures
multiple nucleation sites. Subsequent Y124 crystal growth is along
the crystallographic c-axis, controlled by the chitosan as it decomposes, to form nanowires up to 1000 nm long with an average
width of 50 nm. This is in stark contrast to a small molecule
sol–gel synthesis using acetate and tartrate that produces irregular, micron-sized particles (Fig. 12b and c).
It is also possible to gain control over polydispersity and size
of nanoparticles by using chitosan as a chelating agent, where
the biopolymer matrix slows the sintering process. In this
particular case, aqueous gold and palladium salts were reduced
within a chitosan film followed by heating under argon. In the
absence of air, the chitosan decomposes to carbon with Au/Pd
nanoparticles supported throughout the matrix, showing high
Fig. 12 (a) Structure of chitosan. TEM images of samples of YBa2Cu4O8
synthesized using (b) acetate/tartrate and (c) chitosan. (d) Photograph and
(e) SEM image of chitosan beads prepared by crosslinking with Ce3+ with
(inset) SEM image of sponge-like structure formed after calcination of the
beads. Images reproduced with permission from ref. 11 (b, c) and 109 (d, e).
104 | Mater. Horiz., 2016, 3, 91--112
Materials Horizons
and selective activity for aerobic oxidation of benzylic alcohols.106
The combustion of chitosan from a sol–gel precursor can also be
used to generate pores within a material. For example, gels of
chitosan with sodium silicate were prepared and calcined in air to
produce silica with bimodal porosity, the macroporosity being
generated by removal of chitosan.107
The insolubility of chitosan in basic solutions can be exploited
to add another dimension to sol–gel synthesis in the formation of
gel beads. Aqueous metal salt solutions such as Al(NO3)3 108 or
(NH4)2Ce(NO3)6 109 can be combined with chitosan in acetic acid
to produce homogeneous solutions. Dropwise addition of the
chitosan/metal solutions into aqueous base such as NH4OH
results in gel spheres from precipitation of the chitosan. These
can be calcined in air to produce sponge-like porous spherical
CeO2 or mesoporous Al2O3 (Fig. 12d and e).
5.2.5 Alginate. Alginates are sourced from brown seaweeds
and are a series of linear, unbranched polysaccharides consisting
of b-(1 - 4)-D-mannuronic acid (M) and a-(1 - 4)-L-guluronic
acid (G) residues. Alginate is not a random copolymer, but
contains sections of polymannuronate (–MMMMM–), polyguluronate (–GGGGG–) as well as regions of alternating G
and M. The G : M ratio depends on the seaweed source and
can vary from B30% G to B70% G. Although mannuronate
and guluronate only differ in configuration at the C5 position,
the conformation of the two monomers is very different leading
to polymannuronate segments having a flattened ‘sheet-like’
appearance and polyguluronate forming buckled chains. Alginate is an anionic polysaccharide, each monomer containing a
carboxylate moiety, and so binding to metal cations is strong.
This is particularly the case for polyguluronate segments, which
are crosslinked by multivalent metal cations with each cation
bound to 4 guluronate monomers (known as the ‘egg-box’
model, Fig. 13a). In this way, two polyguluronate segments
can be joined into a left-handed double helix complexing many
metal cations. Alginate salts can be purchased in various forms
(e.g. sodium or ammonium salt) that dissolve in water to form
viscous solutions. Alginic acid is insoluble in water and needs
to be converted to a salt by addition of base. One challenge in
the use of alginate is the strength of the metal–biopolymer
gelation. The strong crosslinking means that the addition of
multivalent cations to alginate results in a rubbery gel that
expels water if the concentration of the two solutions is high.
When using alginate in sol–gel synthesis of ternary or quaternary ceramics, it is therefore important to premix the metal salts
to ensure homogeneity within the alginate.
A search of the literature for ‘alginate sol–gel’ will result
primarily in references for synthesis of aqueous alginate gels
for biomedical applications such as protein or cell encapsulation, drug delivery, or tissue engineering. Many of these exploit
the sol–gel transition of alginate when it is acidified or treated
with metal ions such as calcium. However, there are also many
examples of alginate being used to prepare sol–gel precursors
for materials synthesis. This can either involve direct addition
of aqueous metal salts to sodium or ammonium alginate
solutions, or preparation of a calcium alginate gel template
followed by solvent exchange and infiltration with precursors
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Fig. 13 (a) Egg-box model of cation binding in alginate. TEM images of (b) YBCO nanowires synthesized from alginate with acetate/tartrate precursors
with (inset) BaCO3 nanoparticles that act as sites for catalytic float outgrowth. TEM images of (c) straight nanowires growing from a precursor of alginate
with metal nitrates with (d) schematic showing nanowire spontaneously broadening as a result of microcrucible creep and expansion during heating.
Images modified with permission from ref. 91 and 114 (c/d). Image (b) reproduced with permission from ref. 111. Copyright (2013) American Chemical
Society.
such as titanium alkoxide.110 In the case of gel templates, one
advantage is that the alginate can be prepared in various forms
such as small beads. The resulting metal oxides retain the bead
shape but with a mesoporous structure (Fig. 13b) which could
be particularly useful in applications such as drug delivery111 or
sorption.112
In a similar method to the chitosan synthesis of YBa2Cu4O8,
alginate has been used to prepare nanowires of YBCO superconductors by mixing Y, Ba and Cu acetate/tartrate salts with
alginate to form a gel and then heating to 920 1C in air. As has
been mentioned above, the main limitation in the synthesis of
YBCO ceramics is the uncontrolled nucleation and growth of
BaCO3 crystals, which leads to inhomogeneity in the final
product. In the alginate synthesis, the strong sequestration of
metal cations from aqueous acetate precursors by the biopolymer leads to controlled nucleation of barium carbonate
nanoparticles. A mechanistic investigation on quenched samples
then showed that these barium carbonate nanoparticles act as
catalysts for the outgrowth of YBCO nanowires.113 A more recent
study actually observed nanowire growth in real time using
transmission electron microscopy. In this case, nitrate salts were
combined with alginate and resulted in similar controlled nucleation of barium carbonate but in a more porous Y/Cu matrix. The
rough surface of the matrix provided sites for a microcrucible
mechanism of YBCO nanowire outgrowth (Fig. 13c and d).114 It is
interesting in these cases how a simple change of metal counterion can lead to such different morphologies (tapered wires from
acetates and straight wires from nitrates). When the YBCO phase
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begins to form (4850 1C), the biopolymer has long since been
combusted. However, the way that the biopolymer and the metal
salts interact in the early stages of the synthesis (o400 1C) can
exert remarkable control over growth of the final quaternary
phase. In a subtle departure from the central theme of sol–gel
chemistry, the biopolymer in this case is not ensuring homogeneity but rather controlling inhomogeneity.
As well as metal counterions, the nature of the alginate salt
can also affect ceramic structure. For example, La0.67Sr0.33MnO3
(LSMO, a colossal magnetoresistant material) can be synthesized in the form of nanowires or nanoparticles from sodium
alginate and ammonium alginate respectively.115 In the case of
sodium alginate, a sodium carbonate phase was identified as a
secondary phase and this was proposed to act as a flux, aiding
the transport of other components during the formation of the
LSMO nanowires. Sodium alginate is also compared to sodium
ascorbate in this paper. Alginate and ascorbate have almost the
same empirical formula (C6H7O6 and C6H6O6 respectively) and
so enable a direct comparison of a small molecule and a
polymer. The difference is stark. Ascorbate mixed with La, Sr
and Mn salts results in small but irregularly-shaped particles
compared to the nanowires formed from sodium alginate.
Alginate can also be used in the sol–gel synthesis of metal/
metal oxide nanocomposites and in this case, the alginate has a
dual function. For example, if aqueous Ce(NO3)3 and HAuCl4
are mixed with sodium alginate and dried at room temperature,
the gel turns a bright fuchsia pink as it dries to a flexible film.
Calcination in air at 600 1C then results in a brittle, pink/purple
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solid with a sponge-like network of CeO2 nanoparticles
(diameter B20 nm) with embedded Au nanoparticles.116 In
this synthesis, the alginate firstly binds to Au3+ and Ce3+ cations
via the carboxylate side-groups. The Au3+ is then reduced to Au
via oxidative decarboxylation of the alginate with the polymer
stabilising the resulting nanoparticles. During the calcination
step, the alginate then controls the nucleation and growth of
CeO2 around the Au nanoparticles resulting in a composite
from a single precursor.
A final important point about alginate is the possibility for
tuning the material synthesis through the natural variability of
the biopolymer. Different seaweed species are harvested on a
large scale to extract alginates with different G : M ratios,
primarily for use in the food industry and biomedical applications. In sol–gel synthesis, this has been exploited to control
particle growth in Co, Ni and CoNi nanoparticles.117 In this
system, the alginate with the highest G-content (i.e. providing
the strongest metal binding) produced monodisperse spherical
nanoparticles (B2 nm) whereas medium and low G alginates
lead to larger, less well-defined particles.
5.2.6 Gelatin. Gelatin is a heterogeneous mixture of polypeptides that is prepared by hydrolysis of collagen from animal
skin and bones under acidic or basic conditions, denoted type
A or type B gelatin respectively. The amino acid composition
and peptide chain length can vary considerably depending on
the source. Typically, each strand of the gelatin chain has a
Materials Horizons
molecular weight of B100 000 with a third of the amino acids
being glycine, 21% proline and hydroproline, 10% alanine
and the rest being amino acids in much smaller quantities
(Fig. 14a).118,119 Gelatin dissolves readily in hot water, forming
clear (often pale yellow) solutions with viscosity depending on
the loading, source and molecular weight. On cooling, the
polypeptide chains arrange themselves into left-handed helices
which in turn form a right-handed super helix of 3 strands.
These junction zones, which are usually rich in proline and
hydroxyproline monomers, are what trigger the sol to gel transition in gelatin. Gel strength is a common factor for characterizing
different gelatins and these are quoted as ‘Bloom strengths’.
The diverse range of side-chains on a gelatin molecule make it
a useful gelling-agent for sol–gel chemistry. For example, a mixture
of aluminium, yttrium and terbium nitrates mixed with gelatin in
hot water can be cooled to form a gel. The authors in this study
then infiltrated the gelatin with ammonia to drive precipitation of
amorphous hydroxide intermediates within the gel, before drying
under vacuum and calcining in air to produce fine powders of
YAG:Tb (Terbium-doped yttrium aluminium garnet).120 The homogeneous gelatin precursor ensures a small particle size and size
range (B40–55 nm). This offers substantial advantages over solidstate synthesis as morphology and purity are critical for the
application of YAG:Tb in scintillation and CRT projection.
Gelatin has also been used to produce nanoparticles of metal
nitrides and carbides, as well as oxide/carbide or oxide/nitride
Fig. 14 (a) Typical structure of a gelatin fragment. Images of (b) foam from gelatin and iron nitrate and (c) film from gelatin and iron acetate.
(d) Synchrotron powder X-ray diffraction data showing the iron nitride (+) to carbide (*) transition during a sol–gel synthesis from gelatin. (e) TEM image
of a Fe3C/MgO/C composite showing two types of nanoparticles. TEM (f) and SEM (g) images of a Fe3C/MgO/C composite after acid washing showing
macropores and mesopores. Images reproduced with permission from ref. 121 (c) and 123 (d). Copyright (2010) and (2015) American Chemical society.
Images (e–g) modified with permission from ref. 125.
106 | Mater. Horiz., 2016, 3, 91--112
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nanocomposites. For example, mixing iron nitrate solution with
hot aqueous gelatin results in a viscous, sticky gel that forms an
orange foam (Fig. 14b) on drying at 80 1C in air. In contrast, iron
acetate/gelatin forms a flexible brown film (Fig. 14c) when dried
from a solution. The iron acetate precursor can then be heated to
B800 1C under nitrogen to form Fe3C nanoparticles embedded
in a porous carbon matrix,121 whereas the nitrate precursor
forms Fe3N or Fe3C depending on the heating conditions.122
In situ synchrotron powder diffraction data showed that the
mechanism proceeds via initial formation of iron oxide (FeOx)
nanoparticles, o3 nm in diameter, during the initial decomposition and carbonization stages of the gelatin polymer. The
iron oxide nanoparticles then react with nitrogen species in the
carbon matrix to form Fe3N nanoparticles which then transform
to Fe3C via an intermediate carbonitride (Fig. 14d).123 A similar
approach can also be used to synthesize nanocomposites such as
MIoxide/MIIcarbide (where MI and MII are different metals).
Aqueous or ethanolic metal salts are mixed and combined with
gelatin and the resulting gels dried to foams and calcined under
nitrogen. Despite the homogeneous precursor, the different
thermal stabilities of the metals can drive phase separation to
form composites such as TiO2/Fe3C, TiO2/WN or MgO/Fe3C
(Fig. 14e) with potential applications in catalysis.124 In the case
of MgO/Fe3C, mild acid washing can be used to remove the
nanoparticles to produce carbons with trimodal (macro/meso/
micro) porosity (Fig. 14f and g).125
6. New advances in sol–gel chemistry
This final section highlights some of the recent or unusual
developments in sol–gel chemistry, including synthetic methods
and post-processing techniques. In particular, we focus on alternative physical processing techniques as well as more unconventional or non-aqueous sol–gel precursors.
Review
rapid temperature rise and more uniform heating.128 For
example, a synthesis of barium hexaferrite (BaFe12O19) nanoparticles from metal nitrates, citric acid, EDTA and ammonia
could be achieved by heating freeze dried precursors for just
15 seconds in a modified domestic microwave oven. In this
case, the microwave heating triggers an autocombustion reaction that last for another 10 seconds.129 The shorter heating
time and lower reaction temperature can in turn lead to smaller
particle size and also a narrower particle size distribution. The
authors in this study emphasize the importance of homogeneity in heating and comment that normal heating to trigger
autocombustion in complex metal oxides often leads to mixed
products due to inhomogeneous ignition of hydrocarbons. In
addition, the turbulent flow of hot gases from autoignition
often quickly breaks up samples with resulting particulates
quickly losing temperature, which may also prevent formation
of a desired compound. In this paper, the authors avoid this
problem by containing the autocombustion reaction within a
quartz vessel lined with heat-retaining barium hexaferrite strips.
As previously discussed (section 2.4) hydrothermal synthesis
is another alternative to standard calcination, the high autogenerated pressure and can be used to dissolve or carbonize
organics as well as initiate formation of crystalline inorganics at
much lower temperatures than conventional heating. In the
case of sol–gel precursors, the advantage of hydrothermal
processing is made clear in an article by Fang et al.130 In this
paper, the authors mix iron and tin chlorides with citric acid
and polyethylene glycol in water and ammonium hydroxide to
form a gel. This gel is either dried and treated in a furnace in air
at 350 1C, or heated in a Teflon-lined autoclave at 150 1C. The
result is smaller crystallites of Fe-doped SnO2 in the hydrothermal system, although it should be noted that hydrothermal
treatment requires 12 hours (typical of a hydrothermal synthesis)
compared to the 1 hour at 350 1C in the furnace.
6.2
6.1 Alternative heating processes and lower-temperature
routes
Typically, the conversion of a gel precursor to the desired
inorganic material occurs during a heating step in an air or
inert-gas furnace. However this can lead to a number of problems
such as non-uniform heating, reaction with ceramic crucibles or
container-effects influencing gas escape.126 Furthermore, the slow
cooling rate of conventional furnaces can lead to sintering during
cooling. As such, several authors have developed alternative
heating processes, the most common being microwave heating.
Microwaves have some significant advantages over conventional
heating processes. The electric component of the microwave field
interacts with charge carriers in the material giving rise to direct
heating through dipolar polarization (more significant in the
liquid phase) or conduction heating (dominant in solid materials).
Due to the direct interaction of the microwaves with components
of the material, heating is instantaneous and it also occurs
volumetrically (uniform through the sample).127 In sol–gel
chemistry, this can lead to faster reactions, lower final temperature and even improved phase purity, due to the extremely
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Non aqueous sol–gel chemistry
As discussed at the beginning of this paper, hydrolysis and
condensation of metal alkoxides is strongly dependent on water
content. Most of the other methods of sol–gel processing
described in this paper have been developed as a result of this
limitation of alkoxides. In these methods, the primary focus
has been to form stable chelation complexes and gels from
aqueous metal salts such as nitrates or acetates. However, there
have also been substantial advances in developing alkoxide
chemistry in water-free conditions. This can bring significant
advantages, such as the ability to produce dispersible nanoparticles and also very small particle sizes.
6.2.1 Non-hydrolytic sol–gel (NHSG) chemistry. It is
impossible to cover the breadth of non-hydrolytic sol–gel
chemistry in this paper and in fact there are several excellent
reviews on this field.131,132 Many of the examples in this field
follow similar chemistry to the hydrolysis and condensation of
alkoxides, but necessarily without the hydrolysis step. In nonhydrolytic sol–gel chemistry, the focus is normally still on
formation of a metal oxide. However, rather than coming from
water, the oxygen for the oxide can be sourced from the solvent
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Scheme 6 Condensation steps in non-hydrolytic sol–gel chemistry including (a) alkyl halide elimination, (b) ether elimination, (c) ester elimination and
(d) aldol-like condensation. Modified with permission from ref. 133.
(e.g. alcohol, ether, ketone) or from the organic components
of the precursor (e.g. alkoxides). The key step in formation of
metal–oxygen bonds is normally still a condensation. Rather
than eliminating water, there are several possible condensation
steps such as alkyl halide, ether or ester elimination as well as
an aldol-like condensation (Scheme 6). Alternatively, metal
alkoxides can be heated (normally in a surfactant such as
oleic acid) to decomposition at B250–300 1C via alkene elimination. The resulting hydroxyl groups react with other alkoxides to
form oxo bridges with alcohol elimination. All of these reactions
are described in detail in a review by Debecker et al.133 An
alternative to oxygen-containing functional groups is thio sol–gel
chemistry for producing metal sulfides, as discussed in a review
by Bilecka et al.134
One of the most powerful aspects of NHSG chemistry is the
ability to produce dispersible nanoparticles rather than the
sintered agglomerates typical of many other sol–gel processes.
Non-hydrolytic conditions result in slower and more controllable kinetics than standard hydrolysis reactions, meaning it is
easier to control crystallization. Crystallization of the desired
phase also tends to occur at lower temperatures in NHSG
chemistry. As a result, many NHSG reactions can be carried
out entirely in solvent, with factors such as solvent, temperature and additives having a large influence on structure and
morphology of the crystals. For example, a mixture of iron
carbonyl with dioctyl ether and oleic acid can be heated under
reflux to give monodisperse iron oxide nanoparticles of 6–13 nm
in diameter (with 1 nm increments in diameter) depending on
the molar ratio of precursors.135 Tuning conditions in NHSG
chemistry (such as selective adsorption of surfactants to certain
crystal faces) can also be used to introduce anisotropy into metal
oxides.136
6.2.2 Non-oxide sol–gel chemistry. We have already
discussed the use of aqueous small molecules and polymers
for synthesizing metal nitrides and carbides, typically via
annealing in an inert atmosphere. However, the scope of
sol–gel chemistry for the synthesis of non-oxide materials is
much more extensive and has already been comprehensively
reviewed by Hector.137 Many examples use standard alkoxide
precursors (as in traditional sol–gel chemistry) but produce a
non-oxide material such as a nitride by heating the product in a
reactive atmosphere such as NH3. Alternative approaches focus
on the synthesis of precursors that can be decomposed under
pyrolysis to the desired non-oxide. In this case, the heteroatom
108 | Mater. Horiz., 2016, 3, 91--112
Materials Horizons
is derived from the precursor itself rather than the furnace
atmosphere. For example, early transition metal alkoxides can
be reacted with H2S or hydrazine and the resulting precipitates
heated in an inert atmosphere to produce the corresponding
metal sulfide or nitride.138 Alternatively, many polymeric materials
such as polycarbosilanes or polysilazanes have been used to
generate non-oxide ceramics such as SiC. Like many other sol–
gel methods, the synthesis of non-oxide materials has been
expanded to encompass a wide range of structures and
morphologies such as fibres or films. Considerable work has
also gone into soft and hard templating of non-oxide sol–gel
materials, for example using surfactant micelles or solid ‘casts’
such as anodized alumina membranes to generate mesoporous
ceramics, as reviewed by Shi et al.139
6.2.3 Ionic liquids. Ionic liquids (ILs) are salts that have a
melting point below an arbitrary temperature, normally defined
as 100 1C. They are typically composed of a large organic cation
such as a substituted imidazolium or a tetraalkyl ammonium,
paired with an anion such as nitrate, tetrafluoroborate or
hexafluorophosphate. The physical properties depend on the
composition and can be easily tuned for example by varying the
alkyl substituents on the imidazolium cation. Ionic liquids
have negligible vapour pressure, high thermal stability and
can dissolve a range of polar and non-polar compounds. As a
result, they have been used as solvents and templating agents
in a range of sol–gel synthetic procedures. For example, ionic
liquids have been used to produce aerogels without an energy
intensive supercritical drying step. In this case, ionogels are
first prepared by blending a selected ionic liquid with silica or
organosilica precursors and either water or formic acid. The
ionic liquid anion has been found to be particularly important
in this type of templating synthesis and can be varied to control
the porosity and even surface chemistry of the resulting
silica.140 In many cases, there is interest in the ionogel itself,
as a means of immobilizing an ionic liquid within a solid host
matrix.141 Under the broader definition of sol–gel chemistry as
the synthesis of a material from a molecular precursor, ionic
liquids have also been used to prepare a wide range of porous
or nanostructured carbons.142
As described earlier in this paper, one of the main problems
in the synthesis of ternary and quaternary metal oxides is phase
separation. Even a homogeneous gel precursor may not result
in a phase-pure product, if there is preferential precipitation of
a binary intermediate such as the formation of BaCO3 in YBCO
synthesis. Ionic liquids have been used in an ingenious way
to improve homogeneity during the early stages of ceramic
synthesis.143 This general method has been used to synthesize phase pure samples of a range of metal oxides, such
as YBa2Cu3O7 x, Bi2Sr2CaCu2O8 (BSSCO superconductor) and
yttrium-doped BiFeO3 (multiferroic material) among others.
The method is simple. Aqueous metal salts are added to the
ionic liquid and the mixture heated to remove water. Acetate
and nitrate ionic liquids are selected to enhance metal dissolution within the ionic liquid. Microcrystalline cellulose is then
added and dissolved (ionic liquids are well known to dissolve
cellulose) to provide a polymeric, non-selective chelating agent
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to bind the metal cations within the mixture. Most aqueous
sol–gel syntheses require drying before heating in a furnace
(or quickly dry during the first stages of calcination). The
special feature of this IL synthesis is that the system remains
liquid to a much later stage of synthesis, until the IL decomposes. Future investigations of exactly how the IL decomposes
and controls crystal formation should provide some fascinating
insight into this system.
7. Concluding remarks
The term ‘sol–gel’ has broadened significantly from the original
usage to describe hydrolysis and condensation processes and
many newer methods may not involve a clear sol–gel transition.
Instead, the theme that now seems to connect most sol–gel
chemistries is an exploitation of the solution state in the
synthesis of a solid material. By beginning a synthesis in the
solution (or liquid) state, one can ensure complete homogeneity,
which has considerable advantages in generating a phase-pure
product and can also result in lower synthesis temperatures.
If inhomogeneity is required (for example the formation
of micelles or liquid crystalline phases to template a silica
material), then this is readily controlled to produce well-defined
morphological features in the resulting solid material.
The ability to use molecular precursors to control the
morphology of a ceramic product is one of the main advantages
of sol–gel processing. For example, a simple change of silicon
alkoxide can influence the structure of the resulting silica
monolith. Alternatively, in Pechini synthesis, a simple change
from a diol to a triol can alter the degree of polyesterification
and drive phase separation to produce either spherical or
bicontinuous mesoporous ceramics. Another powerful tool is
the use of additional molecular species that interact with the
sol–gel precursors or each other, often referred to as ‘soft
templating’. In the case where phase separation or structural
ordering happens in the solution state, this is relatively well
understood (although advances in techniques such as small
angle neutron scattering have added new insights into the
templating process).144 What is becoming more apparent is
how changes in molecular composition in the solution precursor
can have a dramatic influence much later on in a ‘sol–gel’
synthesis, for example in the high-temperature synthesis of
ceramics.
Combustion synthesis is perhaps the simplest example of
how a choice of molecular precursor can influence a material
structure even after the solution or gel has been dried and
heated. In this case, a strongly exothermic reaction between an
oxidant such as nitrate and fuel such as citrate or glycine
produces large volumes of gas that result in an open foamlike structure in the ceramic product. The combustion does not
need to be dramatic to produce outgassing and the thermal
decomposition of organic precursors in many sol–gel reactions
also leads to porous solids. In addition to changing the macrostructure, the choice of precursors can also influence the
individual crystallite morphology. For example, in the synthesis
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Review
of YBCO superconductors, a combination of a biopolymer with
metal nitrate precursors results in a porous intermediate
mixture of individual metal oxides and carbonates. The porous
nature of this intermediate facilitates mass transport at the
later stages of synthesis and results in nanowires of the
complex metal oxide product.107 It is even possible to change
the profile of crystallographic transformations in a ceramic
synthesis through a choice of molecular precursors. For example, metastable iron nitride can be favoured over iron carbide
simply through a choice of iron nitrate rather than acetate in
the precursor gel.115 In this case, the higher surface area of the
nitrate-derived precursor results in increases accessibility of the
nitrogen atmosphere that stabilizes the iron nitride phase.
It is clear from all of these examples that there is still much
to be discovered in the field of sol–gel chemistry. With the
increasing power of methods to study high-temperature processes
in situ, we are gaining better understanding of how molecular
precursors can continue to influence a sol–gel process long after
complete combustion of the starting molecules. Furthermore,
there is still much to be learned about the solution state and
how molecular interactions and phase separation changes
during the gelation procedure or during the formation of a
solid from a gel.
Acknowledgements
The authors acknowledge the EU (Marie Curie ‘SusNano’), the
EPSRC, the University of Birmingham and the University of
Bristol for funding.
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